A perforating gun is typically used in the oil and gas industry by insertion downhole into a cased borehole for perforating the casing at a location adjacent a hydrocarbon containing formation. Such a gun is generally elongate and includes a number of charges spaced along its length. The charges are oriented such that when detonated, explosive forces are directed radially outwardly through the casing and into the formation.
A gun can be of any practical length but is typically between 0.5 and 10 meters. The overall diameter of a gun must be small enough such that the gun can be inserted into the borehole casing. The number and spacing of charges can vary and is determined by the requirements of a particular site.
A particular type of gun is expendable, i.e., is discarded after use, and this is the type of gun to which the invention described here pertains. Such a gun is also described in co-pending U.S. patent application Ser. No. 08/517,674 filed Aug. 22, 1995, which application names the same co-inventors as named for this invention and the specification of which co-pending application is incorporated herein by reference. The gun includes a charge holder on which the charges are mounted and an external housing. The charge holder with its mounted charges is sealed inside the housing so as to preclude the entry of dampness.
The outer housing of an expendable gun is typically of metal and is usually cylindrical having a circular outer cross-section to generally match the shape of the inside surface of the borehole casing. The housing, or carrier wall is generally between about 0.75 cm (0.3 inches) and about 1 cm (3/8 inches) thick. It is common for the carrier wall to have thinned or scalloped areas each being located to be aligned with a charge. Less explosive force is required to break through the thinned portion of the wall housing wall when the gun is detonated, as opposed to an unthinned portion. The scalloping thus improves the performance of a gun by increasing the amount of explosive force which makes its way through to the borehole casing and the formation.
As known to the inventors of the invention described in this specification, current methods for manufacturing a gun housing is a relatively labor intensive process. Stock pipe is cut to length, transported to a lathe, threads are turned in to a first end of the pipe, the pipe is re-positioned with respect to the headstock of the lathe, and threads turned in to the second end of the pipe. The pipe is then mounted in a clamp of a milling machine and the thinned areas described above are milled into the external side of the pipe wall.
The present invention is directed to improving this currently used system for producing an external housing in the manufacture of an underground gun.